Chemical mechanical polishing (“CMP”) and electrochemical polishing (“ECP”) are often employed in manufacturing of substrates such as semiconductor wafers or integrated circuits having excess metal deposited on a surface, to remove the excess metal and/or metallic compounds from the surface of the substrate at one or more stages in the manufacturing process. In a typical CMP process, a substrate (e.g., a wafer) is placed in contact with a rotating polishing pad attached to a platen. A CMP slurry, typically an abrasive and chemically reactive mixture, is supplied to the pad during CMP processing of the substrate. During the CMP process, the pad (fixed to a platen) and substrate are rotated while a wafer carrier system or polishing head applies pressure (downward force) against the substrate. The slurry disposed between the pad and the substrate surface accomplishes the planarization process by chemically and mechanically interacting with the substrate film being planarized. Polishing is continued until the desired material on the substrate is removed with the usual objective being to effectively planarize the substrate. Electrochemical polishing involves substantially the same materials and procedures, but also includes imposing an electric potential (and also typically electric current) between a substrate and an anode. The substrate surface to be polished usually includes metal or a metallic compound and, at least when the polishing process is through, a dielectric material. Semiconductor fabrication processes such as photolithography have evolved significantly, such that advanced devices having very fine oxide, metal, and other surface features, with sub-0.18 micron geometries, are now being made. Process tolerances are necessarily tighter for these advanced devices, calling for improvements in CMP technology to obtain desired material removal rates while minimizing wafer defects or damage. It is highly important that the surface be rapidly but accurately and smoothly planarized, that conductors not be over-polished, and that metal ion contamination of the dielectric be minimized.
Economic forces are requiring faster manufacturing processes. One approach to increasing polishing rates involved increasing the downward pressure on the wafer carrier in order to increase material removal rates. This approach is no longer used as the requisite downward pressure is too likely to cause wafer damage, such as scratching, delamination, or destruction of material layers on the wafer. Another approach is using larger and/or rougher abrasives, but the defectivity of the wafers is too high. Another approach has involved increasing the amount of oxidizing agent used. This approach is disfavored as the use of increased amounts of oxidizing agents increases material costs and also adds to the handling issues and environmental issues associated with disposing used slurries.
Additional approaches have involved using various combinations of one or more abrasives including various types of abrasives, sizes of abrasives, shapes of abrasives, size distribution of abrasives, and even BET surface area of abrasives, one or more oxidizers, one or more chelators, one or more corrosion inhibitors, one or more solvents, one or more wettability modifiers, one or more rheology modifiers, one or more abrasive particle modifiers, and other chemicals in the slurry. Some combinations include materials which alone or in the combination have short shelf lives, even to the point of requiring using point-of-use mixing techniques, and these combinations are generally disfavored, as they typically complicate CMP in terms of tooling and process control for example, consume more process time, and/or increase costs.
Typical abrasives include ceria, silica, and alumina, though a wide variety of abrasives are known to those of ordinary skill in the art. Modified abrasives are known—see for example co-owned U.S. Pat. No. 6,743,267, the disclosure of which is incorporated herein by reference thereto, which discloses colloidal ceria or silica abrasive that has been modified with boron-containing compound(s).
In order to achieve fast metal removal rates in a CMP and/or ECP process, a variety of combinations of co-oxidants have been produced. Literally dozens of oxidants have been tested (see, e.g., U.S. Pat. No. 5,958,288 for a partial listing), but only periodic acid, hydroxylamine, ferric nitrate, persulfates (including e.g., peroxydisulfates), urea hydrogen peroxide, peracetic acid, aluminum salts, cerium salts, and hydrogen peroxide (“H2O2”) are commonly used. U.S. Pat. No. 4,959,113 claims CMP slurries having synergistic combinations of two or more salts where the cations are selected from ionized elements (i.e., metals) which will not deposit by electroless plating on the metal surface being polished. Metal ion oxidizers create excessive metal ion contamination problems. Historically, only transition metal salts were thought to be problematic, as sodium, potassium, and lithium have a much lower tendency to contaminate dielectric surfaces. Now that the size of features has gotten so small, even sodium and potassium are recognized as problems, though still not as severe as transition metal contamination. While it is advantageous to minimize sodium and potassium, as used herein, “metals” only means transitional metals and rare earth metals. Lithium, sodium, and potassium are not treated as metals with respect to the claims.
There are a large number of soluble metal oxidizers in use despite the issue of metal ion contamination of a substrate. There are a number of CMP slurries that have small amounts of soluble metal oxidizers. U.S. Pat. No. 5,084,071 described a CMP slurry which contained abrasive particles and a transition metal chelated salt (e.g. EDTA) as a polishing accelerator. Recent patents that disclose CMP slurries having a small amount of soluble transitional metal salts, typically iron salts, which function as oxidizers or polishing accelerators include: U.S. Pat. Nos. 6,491,837, 6,632,377, 6,541,384, and the like. There are scores of references that teach polishing with a CMP composition comprising one or more transitional metal salts, typically ferric salts, but less often aluminum and other salts, in concentrations that intersect the limits of our invention as described in the claims. There is also a growing interest in CMP slurries comprising very small concentrations of soluble rare earth metal salts, typically cerium salts. Such slurries have long been known to be useful in polishing glass—U.S. Pat. No. 4,769,073 describes a CMP slurry for organic glasses having particulate ceric oxide and at least 0.2% by weight of a water soluble cerous (III) salt based on the weight of ceric oxide. Such CMP slurries now find use both in polishing particular metal surfaces and also in polishing dielectric surfaces. Patents that describe CMP slurries having a small amount of soluble rare earth salts include: U.S. Pat. Nos. 6,797,624, 6,399,492, 6,752,844, and 5,759,917.
Of all the oxidants in commercial use, hydrogen peroxide is particularly preferred because of its low cost, and it is benign from the standpoint of product stewardship, as the byproduct is water. Organic oxidizers such as peroxides, hydroxylamines, periodates, persulfates, and the like are prone to lose strength over time, thereby adding a complicating factor to anticipated polishing rates, and ultimately affecting shelf life and bath life. Also, often a straight organic oxidizer is a poor oxidant for selected metals, e.g., hydrogen peroxide for tungsten. The industry has incorporated additives, especially a small amount of soluble metal salt oxidizers, increase the tungsten removal rate during CMP with hydrogen peroxide. There are a number of patents which mention or direct combining organic oxidizers with metal salt oxidizers, and they usually teach minimizing soluble metal ions. U.S. Pat. No. 3,293,093, which is directed to hydrogen peroxide-based etching solutions for copper, states that copper ions, “form active metal ions which have been found to have a highly depreciating effect on hydrogen peroxide (so) that it is rapidly exhausted” These investigators noted that a “catalytic amount” of silver ions, and preferably also a small amount of phenacetin, gave “exceptionally fast etch rates significantly greater than when either additive is used alone.” The patent taught “as little as 10 parts per million” of silver ions is effective, and “about 50-500 parts per million of free silver ion generally represents the preferred amount.”
Other exemplary patents that describe CMP compositions having organic oxidizers (usually per-type oxidizers) and soluble metal oxidizer salts which function as polishing accelerators include: 1) U.S. Pat. No. 5,354,490; 2) U.S. Pat. No. 5,527,423; 3) U.S. Pat. Nos. 5,958,288, 5,980,775, 6,068,787, 6,083,419, 6,136,711, 6,383,065, and 6,527,622; 4) SU 1629353 describes a CMP composition for aluminum alloys containing iron chloride and sodium perborate in the presence of diethyldithiophosphoric acid and ninhydrin; 5) WO 99/53532; 6) U.S. Pat. No. 6,604,987; 7) U.S. Pat. No. 6,783,434; 8) U.S. Pat. No. 5,916,855; 9) U.S. Pat. No. 6,689,692; and 10) U.S. Pat. No. 6,589,100.
While admixing organic oxidizers with soluble metal salt oxidizers addressed the issue of polishing rates, the presence of soluble metal ions exacerbates another problem—metal ion contamination of the substrate, and particularly of the dielectric portions of the substrate, which cause shorts and other electrical phenomena that degrade the product performance. Raghunath et al showed in Mechanistic Aspects Of Chemical Mechanical Polishing Of Tungsten Using Ferric Ion Based Alumina Slurries, in the Proceedings of the First International Symposium on Chemical Mechanical Planarization, 1997, that alumina slurries containing ferric salts is conducive to the formation of an insoluble layer of ferrous tungstate on tungsten. These metal ions migrate and change the electrical properties of the devices, and reduce the reliability of the integrated circuits with time. We found in controlled experiments that even slurries having only a small amount of iron salts, e.g., a slurry having several percent hydrogen peroxide with ˜0.05% ferric nitrate, resulted in dielectric surface iron residues on a wafer of about 10+12 atoms/cm2, compared to the industry goal of metal contamination levels below 10+10 atoms/cm2.
A novel class of CMP compositions described in co-owned published U.S. Applications 20040029495, 20040006924, and 20030162398 that have an abrasive, preferably silica, with a surface coating of a transition metal, preferably iron or copper, capable of initiating a Fenton's reaction with the oxidizer, preferably hydrogen peroxide or periodic acid. The slurry provides greatly increased polishing rates of tungsten with much lower total concentrations of iron and with much lower contamination of the wafer, compared with the peroxide/soluble ferric nitrate slurry described above. Having iron bound to the surface of the abrasive particles reduces transition metal, e.g., iron, contamination considerably compared to the contamination remaining after polishing with soluble metal catalyst. Polishing under substantially identical conditions as those described in the prededing paragraph with an iron-coated silica slurry left iron residues on a wafer at ˜5 times 10+10 atoms/cm2. While this was about 20 times less than the contamination caused by the soluble iron-catalyzed slurry, greater improvements could be made.
The utility of ascorbic acid has been recognized in removing metal ions and abrasives from cleaned and polished surfaces, such as by use of post-etch or post-polish rinses described in U.S. Pat. Nos. 6,635,562; 6,723,691, 6,546,939, 6,156,661, 6,464,568, 5,981,454, 6,326,305, and 6,492,308.
Patents that disclose use of ascorbic acid in any CMP environment include: U.S. Pat. No. 6,806,193 describes using a solution containing ascorbic acid to precondition a polishing pad, but does not describe using ascorbic acid in a CMP composition; U.S. Pat. No. 6,786,945 which includes ascorbic acid in an indistinguished list of hundreds of additives for a slurry of particles having at least one of a rare earth metal hydroxide such as cerium hydroxide and and a tetravalent metal hydroxide such as zirconium hydroxide, where no oxidizer is mentioned; U.S. Pat. No. 6,758,872 which lists ascorbic acid as an additive useful in a polishing slurry which has a peroxide but which does not contain soluble metal salts; U.S. Pat. No. 6,702,954 which describes slurries that can have peroxides and metal salt oxidizers, where ascorbic acid is only mentioned in two slurries in the examples that contain hydrogen peroxide but no soluble metal salts; U.S. Pat. Nos. 6,660,639 and 6,508,953 which describes slurries that have ascorbic acid and an oxidizer selected from peroxides, persulfates or peroxydiphosphates, and further references an application which teaches adding metal salts to such slurries, but requiring the amount of ascorbic acid be high enough to retard corrosion of copper lines, e.g., 0.1% to 1% by weight of ascorbic acid. U.S. Pat. Nos. 6,656,022 and 6,656,021 describe slurries made of particles of soluble metal salts, where ascorbic acid is listed as an otherwise undistinguished acid useful in manufacturing solid organic abrasive particles, but there is no teaching of using soluble ascorbic acid in combination with soluble metal salts; U.S. Pat. Nos. 6,645,051, 6,258,140, 6,117,220, and 6,423,125 describe a polishing composition for a substrate to be used for a memory hard disk containing an organic acid (one of which was ascorbic acid) or a metal salt such as ferric nitrate, but these compositions are used to polish memory hard disks where larger amounts of metal ion oxidizers are used and metal ion contamination is not a factor (further, these patents describe using ascorbic acid or metal salts, and not the two in combination); U.S. Pat. No. 6,348,440 describes compositions for polishing stainless steel, and teaches using iron salts to catalyze hydrogen peroxide; U.S. Pat. No. 6,620,215 which describes CMP slurries having ascorbic acid and hydrogen peroxide, but no soluble metal salts; U.S. Pat. Nos. 6,546,939, 6,156,661, and 5,981,454 describing using ascorbic acid in a post clean treatment, but also describes the composition as useful for polishing copper, though the CMP compositions have no soluble metal ions; and U.S. Pat. No. 5,700,383 which describes a CMP composition containing an oxidant (water or H2O2), a fluoride salt, an abrasive, and a chelating agent, such as citric acid or ascorbic acid, but with no metal oxidizer.
Published U.S. application 20050003744 describes a CMP slurry having active ceria particles and optionally containing hydrogen peroxide and/or ascorbic acid, but there is no teaching of soluble metal salts in the slurry; Published U.S. application 20050067378 describes compositions to pre-roughen copper for use before polishing, where the composition can comprise ascorbic acid, and one composition for pre-roughening contained a cupric ion source, an organic acid (which could be ascorbic acid), a halide ion, and water, but no combination of ascorbic acid and soluble metal ions are described in any polishing composition; Co-assigned and pending Published U.S. application 20040134873 teaches a polishing composition comprising a hydroxylamine compound, optionally a second oxidizer which can be a metal salt, and optionally an acid selected from a list which includes ascorbic acid, and Co-assigned and pending Published U.S. application 20040140288 teaches an etching composition comprising periodic acid or other organic oxidizers, and optionally an acid selected from a list which includes ascorbic acid, but this application does not describe the particular advantages found with using ascorbic acid in combination with soluble metal salts in a polishing composition.
This invention relates to slurries that use regular amounts of per-type oxidizers with small quantities of metal ion oxidizers/polishing accelerators. There are problems associated with such compositions: first, once admixed the metal and oxidizer will react and the shelf life of the mixed slurry will be reduced; and second, metal ion contamination of the substrate, and particularly of the dielectric portions of the substrate, cause shorts and other electrical phenomena that degrade the product performance. What is needed is a CMP composition that addresses these problems.